Mittwoch, 2. März 2016

Science & Research: The safety of concentrated solar power plants

Dr. Christopher Wright, Global Group of Companies, UK ●

The current article discusses recent findings from the article entitled ‘Monitoring light-ends in thermal concentrated solar power plants’ [1] where safe operation is discussed in terms of a plant’s responsibilities to ensure a safe environment according to the dangerous substances and explosive atmospheres regulations 2002 as well as to ensure they are complying with their insurer’s requirements. The perspective is from that of the thermal fluid used in the plant. 

A heat transfer fluid (HTF) is a fluid used to transfer heat from a heat source to a heat user. In terms of a concentrated solar power (CSP) plant, the source is the sun and the HTF is used to heat water and to generate steam that turns a turbine and creates electricity. The most commonly used HTF in this sector is the eutectic blend of biphenyl-diphenyl oxide (e.g., Dowtherm A, Globaltherm Omnitech and Therminol VP-1; Table 1) [2].

Table 1. Typical physicochemical properties for mineral and biphenyl-diphenyl oxide fluids.

Parameter
Unit
Mineral
Biphenyl-diphenyl oxide
Examples of fluids
Descriptive
BP Transcal N, GlobalthermÒ M, Shell Thermia B
Dowtherm A, GlobalthermÒ Omnitech, Therminol VP-1
Operating range
0C
-10 to 320
15 to 400
Appearance
Descriptive
Viscous clear-yellow liquid with a mild odour
Clear-to-light yellow liquid with a geranium-like odour
Density at 250C
kg/m3
873
1056
Kinematic viscosity (at 40, 1000C)
mm2/s
29.8, 4.5
2.5, 0.97
Auto-ignition
0C
>320
621
Maximum film
0C
330
425
Boiling point at 1013 mbar
0C
365
257
Open flash point
0C
230
123
Closed flash point
0C
210
113


At the International Tribology Conference in Melbourne, Australia A. Jackson surmised that “Deciding whether to choose a synthetic lubricant or a mineral oil is governed by two factors: Can the initially more expensive synthetic save money in the long run or can it provide some key performance characteristic not obtainable with a mineral oil?” [3]. It is perhaps no surprise that eutectic blends of biphenyl-diphenyl oxide are synthetic HTFs and its key attribute is that it can be used up to 400 degrees Celsius, which is significantly higher than a mineral-based HTF which operates to around 320 degrees Celsius. Table 1 compares the typical properties for a mineral-based and biphenyl-diphenyl oxide HTFs. A further differentiating feature between these fluids is their respective purity, with the latter having a lower potential to form by-products when operating at elevated temperatures for prolonged periods. 

It is a fact, however, that all HTFs, irrespective of their chemical formation and composition, will thermally degrade. This will be slower with synthetic HTFs, but still needs to be monitored as one of the by-products of thermal degradation are ‘short-chain hydrocarbons’ which are fuel-like components and present a potential fire hazard. These accumulate in a system, such as a CSP plant, as the HTF degrades and need to be actively removed to maintain a normal and safe operation. So the question raised is, how do you enforce a plant to operate safely? (i.e., short-chain hydrocarbons are being removed from the system). This is usually done to comply with current legislation and as a stipulation from the insurer. 

Legislation

Light-chain hydrocarbons boil at lower temperatures than the operating temperature of the fluid itself. As light-chain hydrocarbons accumulate their boiling temperature falls and the 'flash' temperature will also drop. The boiling of light-chain hydrocarbons can lead to vapours being trapped in the system if it is not properly vented. In addition to the drop in flash point temperature, the auto-ignition temperature will also decrease [4]. 

In Europe any system that operates above the flash temperature of the HTF needs to be managed for potential fire risk and in accordance with ATEX Directives 99/92/EC (ATEX 137 or ATEX Workplace Directive) and 94/9/EC (ATEX 95 or ATEX Equipment Directive) [5]. This is usually done by routinely sampling the HTF and testing flash point temperature, which is an indicator of the potential fire risk. 

Insurance

The insurer of a plant may also define how a HTF and HTF system is managed. For example, FM Global [6] used to stipulate that:

1. The HTF is tested for impurities and/or degradation at least yearly.

2. The HTF manufacturer should be consulted to help determine where in the system to take samples.

3. Samples are sent to the supplier of the HTF, although analysis by independent and on site laboratories is also acceptable.

4. A full internal inspection is test results for the HTF indicate a significant level of impurities or thermal degradation 

Conclusions

Solar thermal fluids thermally degrade with prolonged use. It is imperative that the condition of a fluid is monitored routinely to assess its condition and to safeguard the HTF system. HTFs operate at temperatures above their flash point and need to be monitored according to European legislation. Indeed, light-chain hydrocarbons are a by-product of thermal degradation and are a fuel-like substance with an associated fire risk. As light-chain hydrocarbons accumulate, they present an increased fire risk and needs to be monitored. The easiest way to do this is to sample the fluid and test for flash point temperature in the laboratory. For some plant’s, this is a requirement defined by the insurer, who may also specify what is sampled, how it is sampled, where samples are taken from and what to do in the event of a significant change in impurities and thermal degradation. 


[1] Wright CI. Monitoring light-ends in thermal CSP plants. Filtration + Separation 2016: 53 (1): 40-41. Source: http://www.filtsep.com/view/43774/monitoring-light-ends-in-thermal-csp-plants/.

[2] Dowtherm A product literature and data sheets. Source: http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_0931/0901b803809318e6.pdf?filepath=heattrans/pdfs/noreg/176-&fromPage=GetDoc. Accessed: 27th February 2016. 

[3] Jackson A, Synthetic versus mineral fluids in lubrication. Keynote address to be presented at the International Tribology Conference, Melbourne, Australia, December 2-4, 1987.

[4] Ennis T. Safety in design of thermal fluid heat transfer systems. Symposium series number 155. Hazards XXI 2009; 162-169.

[5] Wright CI, Premel J. Heat transfer system safety: Comparing the effectiveness of batch venting and a light-ends removal kit (LERK). Case Studies in Thermal Engineering 2014: 4; 215–221. Link: http://www.sciencedirect.com/science/article/pii/S2214157X1400029X.

[6] Heat transfer by organic and synthetic fluids, in: Factory Mutual 7-99. Property Loss Prevention Data Sheets 12e19, Source: ftp://cable-129-140-83.b2b2c.ca/sda1/Basement/Mes%20documents/Reconnaissance%202011/Codes_Normes/Factory%20Mutual/DS/7-99.PDF.


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Acknowledgements

The author would like to acknowledge the writing support provided by Red Pharm communications, which is part of the Red Pharm company (please see @RedPharmCo on Twitter).

Please contact the author for reference materials cited in this article. 

Dr. Christopher Wright
Christopher Wright
Global Group of Companies
Cold Meece Estate, Cold Meece, 
Staffordshire, United Kingdom

Phone: +44(0)-7967-230-1555



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